Infrared optical system



Dec. 8, 1964 I R. SNYDER 3,160,700 INFRARED OPTICAL SYSTEM 2 4,, 4, 2 Filed Aug. 18, 1961 3 Sheets-Sheet 1 X '2 d f 3 R Q X 2 Z '7 3 RI 2 4 FOCAL PLANE FIG- I. R6

T R TI 2 T 5 R z 4 3 H x MICRONS IR LENS W 2 INCH F/2.0

SOLID LINES ARE ATMOSPHERIC TRANSMISSION WINDOWS I I e.o-- I I.I|3O I.II4O 50 I.IIOO

BACK FOCAL LENGTH (Inches) INVENTOR. FIG, 2 JOHN R. SNYDER ATTORN EY.

Dec. 8, 1964 J. R. SNYDER 3, 60, 00

INFRARED OPTICAL SYSTEM Filed Aug. 18, 1961 3 Sheets-Sheet 2 SEMl-APERTURE IR LENS (menu) 2 INCH F/2.0

SPHERICAL ABERRATION (menu) FIG. 3.

HALF FIELD IR LENS 2 mcu v2.0

TANGENTIAL COMA uncnss) FIG. 4.

HALF FIELD IR LENS 2 INCH F/2.0

FIELD CURVATURE (INCHES) INVENTOR. FIG. 5. JOHN R. SNYDER ATTORNEY.

Dec. 8, 1964 J. R. SNYDER 3, 60,700-

INFRARED OPTICAL SYSTEM Filed Aug. 18. 1961 3 Sheets-Sheet 3 HALF FIELD IR LENS 2 INCH F/2.0

-.oo| o .00!

LATERAL COLOR (mcues) FIG. 6.

I. DISTORTION IR LENS 2 INCH F/2.0

HALF ANGLE FIG. 7.

INVENTOR. JOHN R. SNYDER FAKM ATTORNEY.

United States Patent Ofiice 3,160,700 Patented Dec. 8, 1964 Navy Filed Aug. 18, 1961, Ser. No. 132,512 2 Claims. (Cl. 88-57) (Granted under Title 35, US. Code (1952), sec. 266) The invention herein described may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

The present invention relates to a lens configuration, and in particular to a multiple-element lens system having special use in the infrarfldregion.

In the field of infrared refracting optics has, for the most part, been given little attention. Accordingly, a single element with an aspheric surface is used when the wavelength region is small or when necessity forces the user to tolerate the large variation in focal point with wavelength. Recent improvements in manufacture have yielded infrared transmitting materials in much larger sizes with greatly enhanced physical and chemical properties, widening the possibilities for infrared developments.

Rapid progress in the field of solid-state physics has included the production of commercially available cells capable of detecting infrared radiation up to wavelengths of 14 microns. It is therefore necessary and desirable that lenses be made available to function efficiently in this region. The design of the lens system of the present invention which is corrected for use in the intermediate and far-infrared regions solves one of the difliculties encountered in the field of infrared detection and imaging.

An object of this invention is to provide a lens system that is corrected for use in the intermediate and far-infrared regions.

Another object is to provide a refractory optical system astigmatically corrected for a field of forty degrees and Fig. 3 is a graph illustrating spherical aberration of the lens system of Fig. 1;

Fig. 4 is a graph showing the tangential coma of the lens system of Fig. 1;

Fig. 5 is a graph showing field curvature of the lens system of Fig. 1;

Fig. 6 is a graph showing lateral color in the lens system of Fig. 1; and

Fig. 7 is a graph showing distortion of the lens system of Fig. 1.

Referring more specifically to the drawings, Fig. 1 shows as an illustrative embodiment of the present invention a triplet lens with a relative aperture of f/2.0. It comprises three axially aligned optical elements or components I, II, and III, each separated by an'air space. Each element is transparent to wave lengths of light in the infrared and substantially opaque to visible light. In the direction from the object space to the image space, or from front to rear, the first component I is a positive silicon element of meniscus form with its surface of shorter radius of curvature R lying adjacent to the object space and that of longer radius of curvature R bounding the lens air space S The second component II is a negative arsenic tri'sulfide element of meniscus form with its surface or shorter radius of curvature R bounding in part air space S and that of longer radius R, bounding in part air space 8;. The third component III is a positive silicon double convex element with its radius R lying substantially adjacent to the radius of less curvature R of element II and bounding in part air space S and its radius R lying adjacent to the image space. Intel-posed in the paths of the rays between components II and III is an interior stop or diaphragm 12. A part of the radius of less curvature R of component II lying substantially adjacent to stop 12 forms air space S and radius R of component III lying substantially adjacent to stop 12 forms air space S The materials of the lens system of Fig. l are shown in Table I below:

covering the infrared region of the spectrum from 2.3 Table 1 microns to 11.0 microns.

It is a further object of the present invention to pro- Lens Material Diameter vide a system which achieves correction over a spectral (Inches) range and angular field not heretofore available in lens refracting systems. -"r 2-550 The exact nature of this invention as well as other ob- 1.308 jects and advantages thereof will be readily apparent from consideration of the following specification relating to the Diameter stop 0.618 annexed drawings in which: f 2

Fig. 1 is an illustrative embodiment in accordance with Field coverage: .-I -I 40 degrees. the invention having an aperture of ;f/2 for wavelengths spectralmnge" in infrared; The specifications of the lens system of Fig. 1 are Fig. 2 is a graph illustrating the back focal length as a own n Table 1 l w:

Table II lf/ l Radil Lens Lens N V AN (Inches) Thickness Air Space (Inches) (Inches) R1=+2A08 I 3.4195015") 132.94 .0182 (2.741.011) T =0.267

R:=+3.653 s m 2.732 1(R:)Ra) =0.820 II 2.3964015 29. 09 .0480 (2.741.011) Tz=0.l40

S:(R4-)Rs) =0.427 Ss(R4 Stop)=0.352 S4(Stop-)Rs) =0.075 Rs=+5.4l2 III 3.4195 (6.51.5) 132.94 .0182 (27-110.) 'Iq=0.160

function of the wavelength of light of the lens system of Fig. 1;

In the above table and on the accompanying drawing 1 refers to the focal length of the lens; the Roman numerals refer to the elements comprising the system numbered from the front of object space; N is the index of refraction calculated at 6.5 microns wavelength; AN is the dispersion; V is the reciprocal dispersive power; customarily It is to be understood that the above-described arrangement is illustrative of the principles of the invention. Other such arrangements can be devised by one skilled in the art without departing from the spirit and scope of the definFd as x* uai Where the NS are h 5 invention as set forth in the appended claims. fractive indices at the wavelengths A A and M; R is the what is claimed radn of curvature of the refractive surfaces and the sub- L A lens System substantially as Specified in the scr1pts to these refer to the surfaces; T refers to the thicklowing table, nesses of the elements and the subscripts refer to the elements in axial alignment from front to object space; S refers to the air spaces between the elements and the elements and the stop, and the subscripts refer to the spaces numbered consecutively from the front; the signs in [f/2] the radii column correspond to surfaces which are convex to the front. Rarlii pe s q I h The V-number for silicon and arsenic trisulfide is de- Ihhs hhcheh hhhhce he fined as It =+2.49s N -1 I T.=0.207 V R,=+3.053

N 2.nt N 11.01: earn- 113 =0.s20

Ra=-2.732 T 14 where the Ns are the refractive indices at the correspondn 0 ing wavelengths. In this manner, the V-numbers for silig ggijggg igg con and arsenic trisulfide are calculated as 132.94 and 4 29.09, respectively. III R 518 TFOJGO The triplet element form of the present invention was S (Si,0]) lls)=0.075 chosen for the lens because the parameters are just suflicient to correct for all basic aberrations. The materials are silicon in the two positive elements and arsenic trisulfide in the single negative element. Germanium may be used as a material in the positive lens and possibly modified Selenium would the be Suitable for the single negm where the first column gives the lens elements in Roman tive element. In the present case arsenic trisulfide is the hhmfif'hls Order h the h which element I only material whose index f f ti rekltes to hat of of SlllCOl'l, element II Is of arsenic tr1sulfide and element silicon so as to result in a fiat field condition and at the is of Slhcoh; R, T and 5 respectively the same time provide the necessary long wavelength transof curvature of the refractive surfaces, the thickness of mission. the elements, and the air space between the elements; the

The infrared bandwidth covered by the present siliconelements, radii and the spaces being numbered consecuarsenic triplet system is approximately twenty times that lively from the f is the focal lenglh the lens and of visible light lenses with the same secondary spectrum, 40 the signs correspond to surfaces which are convex to FIG. 1 illustrative of this invention shows that this lens the m. system differs markedly from its visible-light counterpart 2- An objective lens yst m substantially as specifi d m from the standpoint of front-element diameter, element the following tab Radii Lens Lens N V AN (Inches) Thickness Air Space (Inches) (Inches) In: +2498 I 3.4195 (6.5 132.94 .0182 (21-110,!) T|=0.2G7

S (R2 R3)=0.820 R +3.653 1n= 2.732 II 2.3904 (6.5 .1) 29. 09 .0480 (Bi-11.0) T1=0.140

Sz(R4- R5) =0.427 S;(R -St0p) =0.352 1 R5= +5.412 S (Stop-R )=O.UT5 III 3.4195 0.5 132.94 .0182 tar-11.0,.) T;=0.160

spacings, and stop position. In this invention the element spacings and stop position or diaphragm were used to control coma and astigmatism, as illustrated in the graphs of FIGS. 4 and 5, respectively. The present lens system corrects for astigmatism for a held of 40 degrees and covering the infrared region of the spectrum for about 2.3 microns to about 11.0 microns for relative aperture as large as f/2.0. Power ratios between the positive silicon lenses and the arsenic trisulfide lens were used to flatten the field and correct for variation in focal point with wavelength. The diameter of the front lens was used to minimize vignetting. N0 control existed over the secondary spectrum as is illustrated in FIG. 2, and only partial control existed over the spherical aberration as is illustrated by the graph of FIG. 3. FIGS. 6 and 7 illustrate that lateral color and distortion are well corrected by the lens system of this invention.

in which 1 refers to the focal length of the lens; the Roman numerals refer to the elements comprising the system numbered from the front of object space; N is the index of refraction calculated at 6.5 microns wavelength; AN is the dispersion; V is the reciprocal dispersive power; customarily defined as (N -l) (N, -N, where the NS are the refractive indices at the wavelengths M, A and M; R is the radii of curvature of the refractive surfaces and the subscripts to these refer to the surfaces; T refers to the thicknesses of the elements and the subscripts refer to the elements in axial alignment from front to object space; S refers to the air spaces between the elements and the elements and the stop, and the subscripts refer to the spaces numbered consecutively from the front; the signs in the radii column correspond to surface which are convex to the front.

(References on following page) 5 6 References Cited by the Examiner FOREIGN PATENTS UNITED STATES PATENTS 117,978 7/57 Russia.

2,994,053 7/61 De Waard.

3,002,092 9/61 Cary X JEWELL H. PEDERSEN, Primary Examiner 3,007,051 10/61 Amara et al. 5 EMlL G. ANDERSON, Examiner. 

1. A LENS SYSTEM SUBSTANTIALLY AS SPECIFIED IN THE FOLLOWING TABLE: 